The present invention relates generally to an improved method of geophysical exploration and more specifically to an improved method of stratigraphic analysis for identifying and characterizing changes in geology, lithology, and pore fluid content of the formation materials comprising a stratum in the earth's subsurface.
In the continuing search for hydrocarbons in the earth, geophysicists seek methods for evaluating and interpreting the structure of the earth's subsurface formations as well as the effects of geology, lithology, and pore fluid content on geophysical data in order to relate such affects to the occurrence or presence of hydrocarbons. By way of example, exploration geophysicists have developed numerous techniques for imparting seismic wave energy into the earth's subterranean formations; recording the returning reflected seismic wave energy and processing the recorded seismic wave energy to produce seismic signals or traces. Such seismic signals or traces contain a multiplicity of information, for example frequency, amplitude, phase, etc., which can be related to geology, lithology or pore fluid content of the earth's subsurface formations. Such features of the seismic signals are generally referred to as instantaneous attributes. Additionally, interpretative techniques generally referred to as stratigraphic interpretative analysis have been developed for analyzing seismic data and for identifying and characterizing changes in lithology, geology and pore fluid content of the earth's subsurface formations from recurring patterns of instantaneous attribute associated with reflection events in seismic data. Exemplary of such focus are Quay et al. in U.S. Pat. No. 3,899,768 and Bodine in U.S. Pat. No. 4,779,237.
In spite of such developments, stratigraphic analysis of seismic data can be fraught with serious limitations. Most notable of such limitations is the overwhelming volume of instantaneous attributes which an explorationists is confronted with. A first attempt to lessen the volume of instantaneous attributes is described by Bodine in U.S. Pat. No. 4,633,447. Bodine describes a technique whereby a selected instantaneous attribute is chosen to represent an entire wavelet or reflection event. While such approach can reduce the volume of instantaneous attributes, such an approach results in a display of attributes which characterizes differences in adjacent formations represented by each reflection event. Additionally, due to noise and other contaminating features within the recorded seismic signals, the need for true amplitude seismic data, and the resolving power of seismic waves in the 5-200 Hz range, the information conveyed by individual reflection events, and hence the instantaneous attributes, within a stratum can fail to provide an accurate description of changes in the lithology, geology, or pore fluid content thereof.
In light of these difficulties, the present invention provides a novel method of stratigraphic analysis to overcome certain of its known limitations and to better aid the geophysicists in identifying and characterizing subtle changes in the geology, lithology and pore fluid content of formation materials comprising a stratum in the earth's subsurface.